This invention relates to the control of the surface or "case hardness" of steel parts. More particularly, it relates to control of case hardness quality and associated resistance of steel bearing surfaces to wear abrasion, and deformation.
Low surface hardnesses and commensurately poor wearability factors have resulted chiefly from procedures employed in the manufacture of prior art steel bearing surfaces, and particularly in those of trunnions as employed in universal joint cross members. Such members have been traditionally formed from steel forgings, wherein a common practice has been to heat treat the forging prior to all grinding or other metal removal steps. It is common knowledge that such grinding, buffing, or similar finish machining steps remove, at least in part, several thousandths of an inch of the hardened surface achieved from heat treatment and subsequent quenching operations. In fact, the effect of such post heat treatment machining or metal removal steps has been to remove any retained austenite in such case hardened surfaces. Retained austenite has been regarded as undesirable because of its tendency to be readily transformed into untempered martensite under conditions of work hardening, or even the flexure of parts under conditions of extremely cold temperatures. The general thinking in the industry has been that untempered martensite is to be avoided at all costs, as the latter has been associated with dimensional changes of finished parts, as well as brittleness and associated cracking.
Prior art trunnions have therefore been subjected to grinding steps after heat treatment and quenching procedures to remove substantial portions of case hardened layers typically having only 0 to 5 percent retained austenite. The deliberate avoidance of virtually all untempered martensite in the final product has thus resulted in bearing surfaces having less than desirable case hardnesses, along with associated relatively lower resistances to abrasion and deformation.